\begin{tikzpicture}
    \tkzDefPoints{0/0/A, 4/0/B, 0.5/2.5/D,  2.5/2.5/C}
    \tkzInterLL(A,C)(B,D)  \tkzGetPoint{O}

    \tkzDrawPolygon(A,B,D,C)
    \tkzLabelPoints[above](C,D)
    \tkzLabelPoints[below](A,B)
    \tkzLabelPoints[left,xshift=-.3em](O)

    % OA < OB
    % \tkzCalcLength(O,A)  \tkzLabelSegment(O,A){$\tkzLengthResult$}
    % \tkzCalcLength(O,B)  \tkzLabelSegment(O,B){$\tkzLengthResult$}
\end{tikzpicture}

